**Appendix**

(dip2) is selected to monitor the curvature variation trend, corresponding to the resonant wavelength of 1570 nm [15]. The intensity of the actual wavelength decreases as the curvature increases, as shown in **Figure 13**. The intensity of the actual resonant wavelength decreases as the curvature increases, as shown in **Figure 13(a)** [15]. The actual wavelength circled by the red ellipse is about 1571 nm, and the inset is the enlarged view of the intensity variation [15]. The actual wavelength circled by the red ellipse is about 1571 nm, and the illustration is an enlarged picture of intensity change [15]. **Figure 13(b)** reflects that there is also only intensity variation without wavelength shift. The Gaussian fitting resonant wavelength of dip2 undergoes intensity decreasing when the curvature increases from 0.94 to

*Electromagnetic Propagation and Waveguides in Photonics and Microwave Engineering*

*(a) The intensity variation of the actual resonant wavelength with the curvature increased and (b) the intensity variation of the Gaussian fits resonant wavelength (dip2) with curvature increasing from 0.94 to 2.1 m*�*<sup>1</sup> [15].*

In summary, this chapter introduces the optical fiber sensors based on ARROW.

According to the working principle, the optical fiber sensors based on ARROW consist of the single layer, double layers, double resonators, and hybrid mechanism. Various optical fiber sensors based on ARROW have been introduced in this chapter with the aforementioned working principle, including the fiber optic vibration sensor, humidity sensor, strain sensor, temperature sensor, magnetic field sensor, biosensor, etc. The optical fiber sensors based on ARROW could enhance the interaction between the guided light and sensitive material, simplify the complexity of the sensor configuration, and increase the multifunctional performance of the fiber sensor. Especially, many long-standing challenges in the fiber optic sensor can be solved through the working principle of the ARROW, including the temperature cross-talk compensation, distribution localization, and optofluidic biosensing. In general, the optical fiber sensors based on ARROW have advantages, such as compact structure, high sensitivity, large dynamic range, and high stability, which appear to have potential applications in researches of structure health monitoring,

The authors acknowledge the China National Key R&D Program

(Nos. 61835002, 61675033, 61727817, and 61601436).

(No. 2019YFA0706304) and the National Natural Science Foundation of China

2.10 m�<sup>1</sup> [15].

**Figure 13.**

**4. Conclusion**

oil exploiting, and biology detection.

**Acknowledgements**

**24**

#### **Figure 1A.**

*(a) Transmission spectrum of the dual-optofluidic waveguide ARROW biosensor, (b) wavelength shifts with different RI, (c) relationship between the wavelengths of resonance dips and RI, and (d) wavelength interval with different RIs [12].*

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